Came here to post the same thing. I don't know if it has been fixed in 5.4, though.

Nope, the parser seems to be pretty brittle around this. Someone who knows more about the internals can probably explain why this works:

  class User
  {
      public $mapping = array(
          'username' => array(
              'type' => 'string',
              'length' => 32,
              'unique' => true,
          )
      );  

      public function __construct()
      {
      	$this->mapping['nullable'] = function() { echo "test"; };
      	$this->mapping['nullable']();
      }
  }

  $user = new User();

And this doesn't:

  class User
  {
      public static $mapping = array(
          'username' => array(
              'type' => 'string',
              'length' => 32,
              'unique' => true,
          )
      );  

      public static function set()
  	{
  		self::$mapping['nullable'] = function() { echo "test"; };
  		self::$mapping['nullable']();
  	}
  }

  User::set();

I used to be interested in helping fix PHP, but I gave up.

The reason is because Rasmus and a few others seem to think that using the syntax definition to do your type checking for you is a good idea, and that the hundreds of possible cases they didn't think of, don't matter.

Don't need to inline the function, it could instead just be a reference to static member of the same class: ... 'nullable' => self::getNullable, ...

'nullable' => self::getNullable

also won't work in any version of PHP